With the development of the display technology, the liquid crystal display panel possesses properties of small volume and light weight, low power consumption. Therefore, it has been favored by the users. The liquid crystal display device generally comprises the backlight module and the liquid crystal display panel. The backlight module is employed to provide the light for the liquid crystal display panel, and the liquid crystal display panel is employed to show information of words and images. The backlight module generally comprises a light emitting unit, and the light emitting unit receives the supply voltage to emit light. According to the actual requirement, the light emitting brightness of the light emitting unit will be adjusted. However, in prior art, even the light emitting brightness of the light emitting unit is adjusted to be the disclosed brightness, the power consumption of the light emitting unit remains to be larger. Thus, it makes the energy consumption of the liquid crystal display device higher.